Color document processing systems typically include an input device such as a computer workstation that is equipped with document applications for creating, editing and storing electronic documents and an output device such as a printing system for rendering documents. The computer workstation is operated by a user to create, edit or view "softcopy" color images on the color monitor of the workstation. The user can subsequently generate "hardcopy" reproductions of the softcopy images by instructing the workstation to provide the processed electronic image to a selected color printing device. Current advances in color printing technology are helping color document processing systems become more pervasive in desk-top publishing and business applications. Ink jet, thermal-transfer and xerographic color are examples of printing technologies that are making moderate resolution color affordable for these applications. Although more affordable, these printing technologies have some limitations which may surprise and disappoint a user.
Electronic images processed by the workstation consists of a two dimensional array of picture elements (pixels). The color of each pixel may be represented in any of a variety of color notations or color spaces. The colors of softcopy color images are typically defined using a device dependent color classification space such as the additive red, green and blue (RGB) phosphor color space. More specifically, each pixel of the monitor's display contains three primary color phosphors. To generate a color defined by a set of RGB values, the monitor stimulates each primary phosphor with an intensity determined by the corresponding R,G,B value. To be printed, these images need to be converted to the subtractive cyan, magenta, yellow and black (CMYK) or (simply the CMY) color space, which is typically used to put colored dyes, inks, or toners on paper.
Document processing systems typically contains predetermined transform definitions for converting an image defined in one color space to another color space. These transformations are typically defined using a look up table (LUT), that enables a color to be readily mapped from one space to another. Accordingly, the color of each pixel of an electronic image is sequentially mapped using a LUT transform definition to yield a hardcopy representation of a softcopy image. To perform other image transformations that perform functions such as enhance or sharpen a color, the system remaps the color values to yet another point in accordance with another transform definition. Any number of transformations can thus be performed by sequentially mapping color values according to the available predetermined transform definitions.
Transformations used to convert softcopy images to hardcopy images, however, are limited by the color gamuts afforded to softcopy displays and to hardcopy reproduction systems. For example, because of physical limitations of a printing system, such as its resolution, a softcopy of a color image may lose detail when reproduced as a hardcopy. Since the hardcopy reproduction may not have sufficient resolution to reproduce a softcopy image exactly as represented on a color monitor, the printing system may sacrifice appearance detail of a softcopy image in order to preserve its color fidelity. One instance of this limitation is the production of fine lines and text for certain colors. Unlike CRT displays that tend to have many intermediate color shades, most printing technologies are binary in nature, marking with either full ink or none at all. These printing technologies consequently reproduce intermediate shades and tints with a halftone pattern of solid dots. Thus, when a fine line is drawn using such a dot pattern, gaps or stripes in the line may result. For example, small text may have its boundary so disrupted by a halftone dot pattern that the hardcopy rendering of it may be illegible.
Since color printing is performed using a gamut of colors that includes tints and shades of the full color spectrum (e.g. reds, greens, blues and their combinations), printing colored text or fine line graphics on a moderate resolution printing system is difficult for all but a few solid colors. Thus, because of physical device limitations, such as a printing system's resolution, many softcopy color images are inadequately rendered as hardcopy. Specifically, there exists many colors for which lines and text look fine as softcopy on a CRT display but are unacceptable when printed as hardcopy on intermediate resolution printing systems. There exists therefore a need to provide image transformations or mappings that preserve the appearance detail of softcopy images apparent when displayed on color monitors and lost when reproduced as hardcopy on color printing systems. Text, for example, is useless if not legible. Consequently, these transformations should go so far as to sacrifice the color fidelity of the text in order to insure that its appearance detail is preserved.
By way of background, communication protocols between devices such as workstations and printing systems are well known. Some of these protocols define how systems should integrate across networks to provide users with an environment for color document processing. In such an environment communication between devices is transparent to users as a result of the various network protocols that define the manner in which devices exchange information. Specifically, document processing systems can be integrated using Ethernet.TM. and the Xerox Network Systems Communication Protocols which include: Internet Transport Protocols: Xerox System Integration Standard, Xerox Corp., Stamford, Conn., December 1981, XSIS-028112; Courier: The Remote Procedure Call Protocol, Xerox System Integration Standard, Xerox Corp., Stamford, Conn., December 1981, XSIS-038112; Clearinghouse Protocol, Xerox Corp., Stamford, Conn., April 1984, XSIS-078404; Authentication Protocol, Xerox Corp., Stamford, Conn., April 1984, XSIS-098404; and Filing Protocol, Xerox Corp., Stamford, Conn., May 1986, XNSS-108605.
Also, protocols establishing how to encode electronic documents for transmission between various workstations and printing systems using communications protocols is well known. For example, documents can be encoded using a page description languages (PDL) such as Interpress.TM. as disclosed in "Interpress.TM.: The Source Book", Simon & Schuster, Inc., New York, N.Y., 1988, by Harrington and Buckley. In combination with Interpress, the Color Encoding Standard, Xerox System Integration Standard, Xerox Corp, Palo Alto, Calif., July 1991, XNSS 289107 ("The Xerox Color Encoding Standard"), provides a standard for interchanging electronic color documents among document applications and devices. The Xerox Color Encoding Standard describes three reference color systems that attempt to provide device independent color between devices such as workstations and printers.
As described above, many factors, such as resolution, affect the true appearance of an image rendered by different physical devices. Consequently, The Xerox Color Encoding Standard, suggests using "appearance hints", in addition to a reference color system. Appearance hints provide additional information when describing a color. In particular, one appearance hint provides an ability to indicate that, when reproducing an image, it is more important to a user that the detail of the image is reproduced than its original color fidelity. It is therefore desirable that color document processing systems detecting such an appearance hint, convert or map the colors of elements forming an image with a method that retains the appearance detail of image elements while maintaining as much of the original color fidelity of the image elements as possible.
All references cited in this specification, and their references, are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features, and/or technical background.